WO2020234518A1

WO2020234518A1 - Photocathode with improved quantum yield

Info

Publication number: WO2020234518A1
Application number: PCT/FR2020/000176
Authority: WO
Inventors: Pascal Lavoute; Gert NÜTZEL
Original assignee: Photonis France
Priority date: 2019-05-23
Filing date: 2020-05-22
Publication date: 2020-11-26
Also published as: AU2020278915A1; US20220223364A1; US11676790B2; FR3096506A1; CN113994220A; FR3096506B1; SG11202112989YA; JP2022534059A; IL288262A; BR112021023405A2; KR20220011177A; EP3966843A1; CA3139044A1

Abstract

The present invention relates to an electromagnetic radiation detector comprising an inlet window (310) intended to receive a stream of incident photons, as well as a photocathode (320) in the form of a semiconductive layer. A conductive layer (316) is deposited on the downstream face (312) of the inlet window and a thin dielectric layer (317) is disposed between the conductive layer (316) and the semiconductive layer (320). The conductive layer is brought to a potential below that of the semiconductive layer so as to drive the photoelectrons out of the recombination zone and consequently improve the quantum yield of the photocathode.

Description

DESCRIPTION

TITLE: IMPROVED QUANTUM YIELD PHOTOCATHODE

Technical area

The present invention relates to the general field of photocathodes such as those used in image intensifier tubes or photomultiplier tubes.

State of the prior art

Electromagnetic radiation detectors, such as image intensifier tubes or photomultiplier tubes, detect electromagnetic radiation in a given spectral band by converting it into a light or electrical output signal.

These detectors generally include a photocathode for receiving electromagnetic radiation and emitting in response a flow of photoelectrons, an electron multiplying device for receiving said flow of photoelectrons and emitting in response a flow of electrons, called secondary, and finally a device for output to receive said flow of electrons and convert it into an output signal.

Fig. 1 represents an electromagnetic radiation detector known from the state of the art.

As illustrated in the figure, such a detector, 100, comprises an inlet window made of transparent material, 110, generally of glass, serving as a support for a photoemissive or photocathode layer, 120, made of a semiconductor material. The input window has a front face, 111, intended to receive the flow of incident photons and a rear face, 112 opposite to the front face. The photoemissive layer comprises an upstream face, 121, in contact with the rear face of the entry window and a downstream face 122, from which the photoelectrons are emitted.

The photocathode is brought to a negative potential with respect to that applied to the electron multiplier device 130, the electron multiplier device itself being even at a potential negative with respect to that applied to the output device 140, for example a phosphor screen or a CCD matrix.

The photons arriving on the front face 111, pass through the transparent window 110 and enter the photoemissive layer 120, where they generate electron-hole pairs if they have an energy greater than the forbidden bandwidth of the semiconductor material. The photoelectrons migrate to the downstream face, 122, of the photocathode where they are emitted into a vacuum, before being multiplied by the electron multiplier device, 130, and converted into a light or electrical signal by the output device, 140.

The quantum efficiency of the photocathode is conventionally defined as the ratio between the number of photoelectrons emitted by the photocathode and the number of photons received. The quantum efficiency of the photocathode is an essential parameter of the detector, it conditions both its sensitivity and its signal-to-noise ratio. It depends in particular on the wavelength of the incident photons and on the thickness of the photoemissive layer.

The quantum efficiency can be significantly degraded by the presence of defects at the interface between the photocathode, 120, and the transparent window, 110. More precisely, these defects create surface states trapping the photoelectrons generated in the photocathode. The photoelectrons thus trapped can no longer migrate towards the downstream face of the photocathode and therefore do not participate in the photocurrent generated by the photoelectrons emitted by the photocathode.

This deterioration in the quantum efficiency of the photocathode is particularly noticeable at short wavelengths. Indeed, the higher energy photons interact earlier with the semiconductor along their trajectory within the photocathode. The photoelectrons generated by these photons therefore have a higher probability of being trapped at the interface.

To combat this deterioration of the quantum efficiency, it has been proposed to introduce, at the interface between the input window and the photocathode, an intermediate layer of semiconductor material having a forbidden band wider than that of the photocathode. So, for example, if the photocathode is carried out in a III-V semiconductor material such as p-type GaAs, an intermediate layer of p-type GaAlAs can be introduced at the interface. The larger band gap of GaAlsAs compared to GaAs creates an upward band bending on the photocathode side as can be seen in the band diagram of Fig. 2. Thus, when a photon generates an electron-hole pair near the interface, the photoelectron is extracted out of the recombination zone by the local electric field.

However, this solution cannot be transposed to all types of photocathode, in particular to those made from a polycrystalline material, for example in a bi- or multi-alkaline compound such as SbK ₂ Cs, SbRb ₂ Cs, SbRb ₂ Cs, SbCs ₃ , SbNa ₃ , SbNaKRbCs, SbNaKCs, SbNa ₂ KCs. Due to their polycrystalline structure, these photocathodes do not have a well-defined band diagram and it is difficult to provide an intermediate layer of a second semiconductor material making it possible to obtain the desired band curvature at the interface with the polycrystalline material.

More generally, even for photocathodes made from a monocrystalline semiconductor material, it is not always easy to find a second suitable semiconductor material making it possible both to obtain a mesh match and the desired band curvature with the material. semiconductor constituting the photocathode. This is particularly problematic for photocathodes produced in a II-VI semiconductor material, such as CdTe.

An aim of the present invention is therefore to provide an electromagnetic radiation detector having a photocathode made of a first semiconductor material, exhibiting a high quantum efficiency without however requiring an intermediate layer made of a second suitable semiconductor material, between the input window. and the photocathode.

Presentation of the invention

The present invention is defined by an electromagnetic radiation detector comprising a glass entry window having an upstream face intended to receive a flux of incident photons as well as a downstream face opposite the face. upstream, a photocathode in the form of a semiconductor layer, intended to generate photoelectrons from the incident photons and to emit said photoelectrons thus generated, an electron multiplier device configured to receive the photoelectrons emitted by the photocathode and to generate for each photoelectron received a plurality of secondary electrons and an output device configured to generate an output signal from said secondary electrons, the radiation detector being specific in that a transparent conductive layer is deposited on the downstream face of the window input and that a thin insulating layer is disposed between said conductive layer and the semiconductor layer, the conductive layer being electrically connected to a first electrode and the semiconductor layer being electrically connected to a second electrode, the first electrode being intended to be raised to a lower potential than that applied to the second electrode.

The semiconductor layer can in particular be made from a polycrystalline semiconductor material. This material can be chosen from SbK ₂ Cs, SbRb ₂ Cs, SbRb ₂ Cs, SbCs ₃ , SbNa ₃ , SbNaKRbCs, SbNaKCs, SbNa ₂ KCs.

Alternatively, the semiconductor layer can be produced in a monocrystalline semiconductor material III-IV or II-VI.

The transparent conductive layer is typically made of ITO or ZnO.

The thin insulating layer is advantageously made of a dielectric material having a breakdown voltage greater than 1 V / 10 nm. This thin insulating layer generally has a thickness of 100 to 200 nm. The dielectric material is advantageously chosen from Al ₂ 0 ₃ , Si0 ₂ , Hf0 ₃ .

The potential difference applied between the second electrode and the first

4ô Js _s AU _bb eN _a

electrode is advantageously chosen greater than or equal to where s _s and e

s _d are respectively the dielectric constants of the semiconductor layer and of the insulating layer, d is the thickness of the insulating layer, AU _bb is the amplitude of the band curvature in the absence of applied potential difference, N _a is the concentration of acceptors in the semiconductor layer and e is the charge of the electron.

Brief description of the figures

Other characteristics and advantages of the invention will become apparent on reading a preferred embodiment of the invention, described with reference to the appended figures, among which:

Fig. 1, already described schematically represents the structure of an electromagnetic radiation detector known from the state of the art;

Fig. 2 represents the band diagram of a photocathode with high quantum efficiency, known from the state of the art;

Fig. 3 schematically shows the structure of an electromagnetic radiation detector according to one embodiment of the invention;

Fig. 4 shows the band diagram of a photocathode used in the electromagnetic radiation detector of FIG. 3.

Description of embodiments

The idea behind the invention is to introduce between the entry window of the electromagnetic detector and the photocathode, a capacitive structure formed of a thin conductive layer serving as a polarization electrode, and a thin layer dielectric. The polarization electrode is intended to be polarized at a potential lower than that applied to the photocathode so as to drive out of the recombination zone the photoelectrons generated near the interface. Fig. 3 schematically shows the structure of an electromagnetic radiation detector according to one embodiment of the invention.

As previously, the detector comprises an entry window, 310, made of a material transparent in the spectral band of interest, for example a window made of quartz or of borosilicate glass. The input window has an upstream face, 311, intended to receive the flow of incident photons and a downstream face, 312, opposite the upstream face.

A conductive layer, 316, transparent in the spectral band of interest, is deposited on the downstream face of the entry window. It is also electrically connected to a first electrode 315. The transparent conductive layer can advantageously be made of ZnO or ITO. Its thickness is chosen from the range 50 to a few hundred nm and advantageously equal to 150 nm.

An insulating layer, 317, made of a dielectric material, is placed between the conductive layer 316 and the semiconductor layer of the photocathode, 320. The dielectric material is chosen to have a high breakdown voltage, for example greater than IV / 10 nm. . The thickness of the dielectric layer is typically 100-200 nm. The dielectric material may in particular be alumina (Al ₂ O ₃ ), silica (Si0 ₂ ), or a Hafnium oxide (HF0 ₃ ). According to one variant, the insulating layer may be produced in the form of a multilayer dielectric structure involving the aforementioned dielectric materials.

The photocathode 320 is produced in the form of a semiconductor layer deposited on the insulating layer 317. The semiconductor can be monocrystalline, for example a III-V semiconductor, such as GaAs or II-VI, such as CdTe. Alternatively, it may have a polycrystalline structure, as may in particular be the case for bi- or multi-alkali compounds such as SbK ₂ Cs, SbRb ₂ Cs, SbRb ₂ Cs, SbCs ₃ , SbNa ₃ , SbNaKRbCs, SbNaKCs, SbNa ₂ KCs.

In all cases, the photocathode is electrically connected to a second electrode, 325.

The primary electrons emitted by the photocathode are emitted into a vacuum and multiplied by an electron multiplication device, 330, for example a microchannel wafer (GMC) or a layer of nanocrystalline diamond as described in the published application FR-A- 2961628 filed in the name of the present Applicant, or even a discrete dynode multiplier in the case of conventional photomultipliers. The electron multiplication device is connected to a third electrode (not shown).

The photoelectrons thus multiplied, called secondary electrons, are received by the output device, 340. The output device may include a phosphor screen, ensuring direct conversion into an image as in an image intensifier or else a CCD or CMOS matrix for provide an electrical signal representative of the distribution of the incident photon flux, as in an EB-CCD (Electron Bombarded CCD) or EBCMOS (Electron Bombarded CMOS) system, or a simple metal anode in the case of conventional photomultipliers.

The output device is connected to a fourth electrode acting as an anode.

The input window 310, the photocathode 320, the electron multiplication device, 330 and the output device are advantageously mounted in a compact tube body, the electrical connections of the electrodes with the external power supply being provided by rings. of connection separated by dielectric spacers. According to an advantageous variant embodiment, the tube body may be in the form of a multilayer ceramic substrate on which the electron multiplication device will be fixed as described in the published application FR-A-2925218 filed in the name of present Applicant.

Of course, as in a conventional photocathode, the extraction of the photoelectrons and their acceleration is ensured by applying a high voltage between the anode and the cathode. However, in an original manner, a negative voltage is applied between the first electrode and the second electrode so that the conductive layer is brought to a potential lower than that of the photocathode. More precisely if we denote respectively V _m , V _pk , V _a the respective potentials of the conductive layer, of the photocathode and the anode, we have V <V, “V. In other words, the potential difference between the conductive layer and the anode is essentially due to that between the photocathode and the anode. In practice, the potential difference V _{pk -} V _m will be between 1 and 50 V while the potential difference V _{a -} V _pk is of the order of several hundreds of V. The application of this voltage to the first electrode has the effect of driving the photoelectrons generated in the recombination zone 321 towards the emission surface 322 of the photocathode. The recombination zone of the photocathode is located at the interface with the dielectric layer. Those skilled in the art will in fact understand that the dislocations and defects at the interface with the dielectric layer play the role of photoelectron recombination center. The residence time of the photoelectrons in the recombination zone is very short due to the electric field applied between the conductive layer and the photocathode and correspondingly reduces their probability of recombination.

In addition, the transport of photoelectrons within the photocathode is no longer due mainly to diffusion but also to the internal electric field. This results in a reduction in the average travel time of the electrons in the photocathode and an improvement in the response time of the photodetector.

The conductive layer is indicated by 410, by 420 the insulating layer and by 430 the semiconductor layer of the photocathode.

The upper part of the figure, designated by (A), corresponds to the situation where no potential difference is applied between the conductive layer and the semiconductor layer (p-type).

Note that the conduction and valence bands of the semiconductor layer are curved downward at the interface with the insulating layer. In other words, in such a situation, a photoelectron gas forms at the interface, in the potential cup 424. Moreover, 425 has been designated by the recombination zone where the surface states are located.

Photoelectrons generated at or near the interface have a high probability of recombination with surface states, especially since a photoelectron present in the potential cuvette will tend to migrate towards the recombination zone.

The lower part of the figure, designated by (B), corresponds to the situation where the conductive layer is brought to a potential lower than that of the layer semiconductor. More precisely, the potential difference V - V is chosen here to be greater than a threshold value AV _th as explained below.

Note that the conduction and valence bands of the semiconductor layer are this time curved upwards at the interface with the insulating layer. In other words, in such a situation, the photoelectrons generated at the interface are driven from the recombination zone 425 by the electric field present in the zone of curvature of the bands.

The potential difference V _{k -} V _m to be applied can be estimated as follows: in the absence of applied voltage (situation (A)), the space charge (negative) corresponding to the curvature of bands balances the charge (positive ) surface conditions. This space charge can be approximated by:

where e is the charge of the electron, N _a is the concentration of acceptors in the photocathode (p-type) and x _dt is the width of the depletion zone.

The width of the depletion zone can be estimated by:

4 th _s AU _bb

x (2)

eN _a where s _s is the dielectric constant of the semiconductor and AU _bb is the band curvature in the absence of potential difference. It follows that Q,

.

The potential difference to be applied between the conductive layer and the photocathode making it possible to simply balance this charge by capacitive effect in the photocathode is therefore:

where the FF index corresponds to a situation where the bands are flat at the interface, d is the thickness of the insulating layer and s _d its dielectric constant. If we want to at least reverse the curvature of the bands, it will then be necessary to apply a potential difference

_{, F} 4Sjs, AU _u eN _a

Those skilled in the art will however understand that an improvement in the quantum efficiency will be obtained as soon as V _m <V _pk , insofar as any reduction in the curvature of the bands, even before its inversion, will reduce the width of the potential pit. at the interface and will therefore decrease the probability of photoelectron recombination.

Claims

1. Electromagnetic radiation detector comprising a glass entry window (310) having an upstream face (311) intended to receive a flux of incident photons as well as a downstream face (312) opposite the upstream face, a photocathode under in the form of a semiconductor layer (320), intended to generate photoelectrons from the incident photons and to emit said photoelectrons thus generated, an electron multiplier device (330) configured to receive the photoelectrons emitted by the photocathode and to generate for each photoelectron received a plurality of secondary electrons and an output device (340) configured to generate an output signal from said secondary electrons, characterized in that a transparent conductive layer (316) is deposited on the downstream face ( 312) of the entry window and that a thin insulating layer (317) is disposed between said conductive layer (316) and the semiconductor layer (320), the conductive layer being electrically connected to a first electrode (315) and the semiconductor layer being electrically connected to a second electrode (325), the first electrode being intended to be brought to a potential lower than that applied to the second electrode.

2. Electromagnetic radiation detector according to claim 1, characterized in that the semiconductor layer is made of a polycrystalline semiconductor material.

3. Electromagnetic radiation detector according to claim 2, characterized in that the polycrystalline semiconductor material is chosen from SbK ₂ Cs, SbRb ₂ Cs, SbRb ₂ Cs, SbCs ₃ , SbNa ₃ , SbNaKRbCs, SbNaKCs, Sbl \ la ₂ KCs.

4. Electromagnetic radiation detector according to claim 1, characterized in that the semiconductor layer is made of a monocrystalline semiconductor material II-IV or II-VI.

5. Electromagnetic radiation detector according to one of the preceding claims, characterized in that the transparent conductive layer is made of ITO or ZnO.

6. Electromagnetic radiation detector according to one of the preceding claims, characterized in that the thin insulating layer is made of a dielectric material having a breakdown voltage greater than 1 V / 10 nm.

7. Electromagnetic radiation detector according to claim 6, characterized in that the thin insulating layer has a thickness of 100 to 200 nm.

8. Electromagnetic radiation detector according to one of the preceding claims, characterized in that the dielectric material is chosen from Al ₂ 0 ₃ , Si0 ₂ , Hf0 ₃ .

9. Electromagnetic radiation detector according to one of the preceding claims, characterized in that the potential difference applied between the second electrode and the first electrode is chosen to be greater than or equal to

45d _s AU _bb eN _a

where s _s and s _d are respectively the dielectric constants of the semiconductor layer and of the insulating layer, d is the thickness of the insulating layer, AU _bb is the amplitude of the band curvature in the absence of an applied potential difference, N _a is the concentration of acceptors in the semiconductor layer and e is the charge of the electron.